In high intensity discharge lamps, light is generated when an electric current is passed through a gaseous medium. The lamps have variable resistance characteristics that require operation in conjunction with a ballast to provide appropriate voltage and current limiting means. Control of the voltage, frequency and current supplied to the lamp is necessary for proper operation and determines the efficiency of the lamp. In particular, it determines the size and weight of the required ballast.
The appropriate voltage, frequency and current for efficient running of a lamp in its normal operating stage is not appropriate for the lamp during its warm-up stage. A high intensity discharge lamp typically takes several minutes to warm-up from the time it is struck or turned on to its normal operating state. Initially the lamp is an open circuit. Short pulses of current are sufficient to strike the lamp provided they are of adequate voltage. Subsequent to striking, the lamp's resistance drops radically. The resistance then slowly rises during warm-up to its normal operating level. Hence, subsequent to striking and during warm-up the current of the lamp must be limited to prevent internal lamp damage.
At times during warm-up, high intensity discharge lamps exhibit "bulb rectification". For reasons not completely clear, the lamp temporarily conducts in only one direction. Bulb rectification tends to decrease the useful life of the lamp unless the current to the lamp is quickly reduced. A ballast systems must achieve its objective while reducing the current during bulb rectification.
Certain prior art devices teach the use of current limiters and current detection circuits to control the current to the lamp. For example, U.S. Pat. No. 4,370,601 issued on Jan. 25, 1983 to Horii et al is one such device. U.S. Pat. No. 4,240,009 issued Dec. 6, 1980 to Paul teaches a control circuit for providing constant current to the lamp during warm-up and constant wattage thereafter. U.S. Pat. No. 4,238,710 issued Dec. 9, 1980 to Nelson teaches a voltage feedback control loop to minimize the effect of powerline variations. Similarly, in U.S. Pat. No. 4,415,839 issued Nov. 15, 1983 to Lesea the power consumption level of the lamp is monitored and lamp power consumption is regulated in response thereto.
However, none of the prior art devices recognizes the bulb rectification problem discussed above, and of course they do not suggest any means for solving it. Also, the prior art does not suggest current limiters which are capable of responding during the present duty cycle to further insure that a destructive overcurrent condition, such as that caused by bulb rectification, does not occur.
The Applicant's co-pending patent application Ser. No. 770,663 filed Aug. 28, 1985, and incorporated by reference herein, is an attempt to solve the problems of controlling lamp current during start-up and of bulb rectification. Ser. No. 770,663 uses a means sensitive to the radiant energy or heat of the lamp as a feedback loop to control the lamp's start-up characteristics.